7/27/2019 PH_A_008
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PH-A-008http://www.yokogawa.com/us
Sour Water
Industry: Refining, Oil and GasProduct: PH450
Background InformationSour Water is the wastewater that is
produced fromatmospheric and vacuum crude columns at
refineries.Hydrogen sulfide and ammonia are typical components
insour water that need to be removed before the water can bereused
elsewhere in the plant. Removal of thesecomponents is done by
sending the sour water from theprocess to a stripping tower where
heat, in the form ofsteam, is applied. The ammonia and hydrogen
sulfidecontained in the water is released by the heat and exits
thetop of the tower.
IntroductionThe ideal pH value for stripping H2S is below 5,
since above5, sulfide is primarily found in the form of ions
(HS
-or S
-2).
Alternatively, efficient ammonia stripping requires a pHabove 10
to prevent the formation of ammonium (NH
4+) ion
that cannot be stripped. Although the most favorable
strategy
for sour water stripping is a three step process where
twoseparate stripper towers are used, one for removinghydrogen
sulfide and the other for removing ammonia,economics usually
dictates a compromise. Having only onestripper tower and using a pH
around 8 allows adequateremoval of both gases.
There are three distinct processing steps in the sour
waterstripping process: degasification, hydrogen
(acid-gas)stripping and ammonia stripping. Figure 1 shows only
onestripper column.
During the degasification stage the sour water feed from
theplant is cooled and fed to a degasser where dissolvedhydrogen,
methane and other light hydrocarbons are
removed. These removed gases are known as sour Gas andare pumped
off to the Sulfur Recovery Unit (SRU). Thisdegassed sour water is
pumped into a storage tank thatserves to dampen the flow rate and
facilitates removal ofentrained oil and solids.
The next step in the process is known as hydrogen
sulfidestripping. The degassed sour water is fed to the acid gas
orhydrogen sulfide stripper, which is a steam-reboileddistillation
column. The hydrogen sulfide, which is stripped
overhead, is of high purity an excellent feed for sulfurrecovery
units or sulfuric acid plants.
Next, the hydrogen sulfide stripper stream, containing all
theammonia in the feed water and some hydrogen sulfide, is fed
directly to the ammonia stripper, which is a
refluxeddistillation column. In this column, essentially all
ammoniaand hydrogen sulfide are removed from the water.
Afterexchanging heat with the hydrogen sulfide stripper feed,
thestripped water is cooled and sent off for either reuse
ortreating.
ChallengesMeasuring pH in sour water poses several challenges.
Theabrasiveness of the wastewater affects the pH electrodesover
time; the reference electrode junction may plug andrequire
cleaning; and the reference element itself will bepoisoned over
time due to the presence of hydrogen sulfide.Process temperatures,
which affect sensor life, can be quite
high in order to facilitate removal of the unwantedcomponents.
Ammonia and cyanide can poison thereference electrode by reacting
with the silver element.
Product RecommendationsThere are two recommended pH sensor
combinations werecommend for sour water monitoring. If you are
looking fora retractable installation assembly, the PH87 holder
with thePH97/DP sensor is the best chose. The sensors solid
statereference provides longer lifetime by lengthening the amountof
time it takes for the reference to become poisoned.
For either a flow thru or a direct insertion assembly
werecommend breaking the pH measuring system up into it
three parts: the measuring electrode, reference electrode,and
the temperature electrode. The Yokogawa FF20 is theflow thru holder
and the FS20 is the direction insertionholder, each are available
in different materials ofconstruction. For the temperature
electrode we recommendthe SM60-T1, a Pt1000 thermometer. The
measuringelectrode we recommend is the SM21-AL6, a heavy dutyglass
electrode that is strengthened for chemical resistanceand higher
temperatures. The Bellowmatic SR20-AC32reference electrode is
recommended for the reference
